Surface-coated reduced iron, method for making the same, and method for melting the same

ABSTRACT

A method for making surface-coated reduced iron includes the step of coating the surface of reduced iron with a tar emulsion. Preferably, the tar emulsion includes at least one hydrocarbon-based material selected from the group consisting of natural petroleum tar, coal tar, pitch, asphalt, liquefied coal, and residual oil from petroleum refining; a surfactant; and water. More preferably, the tar emulsion contains 60 to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant, and the balance being substantially water.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a technique for preventingreoxidation of reduced iron, for example, produced by a direct reductionprocess, during storage and transportation, and a technique for meltingreduced iron efficiently.

[0003] 2. Description of the Related Art

[0004] Reduced iron (sponge iron) produced by a direct reduction processhas numerous open pores formed in the direct reducing process, and whenthe reduced iron is exposed in the air for a long period of time, theinner surfaces of the open pores are easily oxidized (reoxidized). Ifsuch reoxidized reduced iron is melted as feed material to meltingprocess, a slag with a high iron oxide content is formed on the surfaceof the molten iron, thus decreasing the yield of iron. Additionally,when the sponge iron having numerous open pores is used, boiling mayoccur during melting, or the carbon content in the molten iron maybecome unstable because a portion of carbon is consumed by the reductionof iron oxides in the reduced iron and in the slag. Moreover, if theiron oxide content in the slag is increased, the viscosity of the slagis decreased, and the loss of the refractory lining of the meltingfurnace is increased.

[0005] Therefore, in order to prevent the reoxidation of the reducediron, various methods have been proposed. For example, JapaneseUnexamined Patent Application Publication No. 50-116343 discloses amethod in which the surface of reduced iron is treated with a mediumcontaining an appropriate amount of oxygen to form a passive filmcomposed of dense oxides on the surface of the reduced iron; JapaneseUnexamined Patent Application Publication No. 52-107248 discloses amethod for preventing oxidation by applying a dispersion liquid, inwhich graphite is diluted with an alcohol or water, to the surface ofreduced iron; Japanese Examined Patent Application Publication Nos.60-17803, 58-3003, 60-15681, etc. disclose methods in which CaO and ironoxides are applied to the surface of reduced iron so that the surface iscoated with hydrates of CaO; Japanese Unexamined Patent ApplicationPublication No. 47-5806 discloses a method in which air or nitrogen towhich ammonia is added is brought into contact with reduced iron;Japanese Examined Patent Application Publication No. 59-17164 disclosesa method in which reduced iron is immersed in a solution of an alkalimetal silicate and is then dried by heating; Japanese Examined PatentApplication Publication No. 60-12404 discloses a method for coatingreduced iron with a hydraulic cement; Japanese Unexamined PatentApplication Publication No. 55-128515 discloses a method for coatingreduced iron with a chromate; Japanese Unexamined Patent ApplicationPublication No. 52-123312 discloses a method for cooling reduced iron ina molten salt; Japanese Unexamined Patent Application Publication No.58-71315 discloses a method for preventing ignition or heating up bycontrolling the discharging temperature after reduction; JapaneseUnexamined Patent Publication Nos. 59-170213, 52-78610, etc. disclosemethods in which reduced iron is processed by hot press molding (hotbriquetting) using dies to decrease its specific surface areas, andthereby oxidation is prevented; and Japanese Unexamined PatentPublication No. 55-119116 discloses a method for preventing oxidation bycontinuous rolling.

[0006] However, except for the hot pressing (hot briquetting) method,the effects of the conventional methods described above are not stable,and the resultant products do not sufficiently last during vesseltransportation and long storage. With respect to the hot pressing (hotbriquetting) method, dedicated facilities are required to perform themethod, and moreover, since heat treatment must be performed at a hightemperature of 800 to 900° C., the facilities are subjected tosignificant wear and tear, resulting in an increase in the maintenancecosts and other operational problems.

[0007] With respect to the methods for forming protective films atambient temperatures to prevent reoxidation, inorganic oxides, such asCaO, calcined lime, cement, water glass, and graphite, are used. In suchmethods, except for the methods using water glass and graphite, rigidoxidation-preventing films are formed using the hydration reactions andalso iron is prevented from being oxidized by maintaining the alkalineenvironment. However, since water glass is highly hygroscopic and isneutralized by reaction with carbon dioxide in the air, the effectthereof does not substantially last long.

[0008] In the method using graphite, by covering the surface of reducediron with a dense graphite film, oxygen is prevented from penetratinginto the reduced iron. However, in practice, it is very difficult toform a dense film using graphite, and the resultant film has manyinterstices. Therefore, it is not possible to obtain a satisfactoryoxidation-preventing effect.

[0009] Additionally, since the graphite film is electrically conductive,even when reduced iron is melted in an electric furnace, meltingoperation can be performed without any problems. However, in the methodsin which the inorganic films other than the graphite film are formed,since the films are nonconductive, the melting reaction of the reducediron is significantly hampered, resulting in a decrease in the meltingefficiency.

[0010] In order to overcome the problems described above, the applicantof the present invention has disclosed a method for coating reduced ironwith an organic film-forming material, such as pitch, asphalt, coal tar,liquefied coal, or coal residual oil (Japanese Unexamined PatentPublication No. 8-260172). However, since most of these materials havehigh viscosities at ambient temperatures, it is difficult to coat thesurface of the reduced iron thinly and homogeneously with the materials,even if appropriately heated. Therefore, in order to form theoxidation-preventing film, the organic film-forming material usuallymust be dissolved or dispersed in an appropriate solvent to decrease theviscosity, and then the surface of the reduced iron is coated by animmersion method, a spray method, or the like, followed by drying underreduced pressure or drying by heating to remove the solvent. Since thesematerials still have high viscosities even if dissolved in solvents, itis not possible to form a completely homogeneous film on the surface ofthe reduced iron, and thus it is not possible to prevent reoxidationsufficiently. Moreover, the solvents to be used are expensive, and forsafety purpose, recovery facilities to recover the volatized solventsmust be installed, resulting in an increase in the cost of equipment.

SUMMARY OF THE INVENTION

[0011] It is an objective of the present invention to providesurface-coated reduced iron having a reliable and long-termoxidation-preventing effect, in which the increased cost of equipmentand the operational problems associated with the hot pressing (hotbriquetting) method are solved and in which the insufficientoxidation-preventing effect and the insufficient durability associatedwith the conventional surface-coating methods are improved. It isanother objective of the present invention to provide a method formaking surface-coated reduced iron, in which the reduced iron can bereliably prevented from being reoxidized for a long period of time bysimple surface treatment. The further objective of the present inventionis to provide a method for melting such surface-coated reduced ironefficiently.

[0012] In one aspect of the present invention, in surface-treatedreduced iron, the surface of the reduced iron is coated with a taremulsion.

[0013] Preferably, the aforementioned tar emulsion contains at least onehydrocarbon-based material selected from the group consisting of naturalpetroleum tar, coal tar, pitch, asphalt, liquefied coal, and residualoil from petroleum refining; a surfactant; and water.

[0014] Preferably, the aforementioned tar emulsion contains 60 to 80percent by mass of the hydrocarbon-based material, 0.1 to 1 percent bymass of the surfactant, and the balance being substantially water.

[0015] In another aspect of the present invention, a method for makingsurface-coated reduced iron includes the step of coating the surface ofthe reduced iron with a tar emulsion.

[0016] In the method for making surface-coated reduced iron, preferably,the aforementioned tar emulsion contains at least one hydrocarbon-basedmaterial selected from the group consisting of natural petroleum tar,coal tar, pitch, asphalt, liquefied coal, and residual oil frompetroleum refining; a surfactant; and water.

[0017] In the method for making surface-coated reduced iron, preferably,the aforementioned tar emulsion contains 60 to 80 percent by mass of thehydrocarbon-based material, 0.1 to 1 percent by mass of the surfactant,and the balance being substantially water.

[0018] In the method for making surface-coated reduced iron, preferably,the reduced iron is produced by reduction with a gaseous reductantcontaining hydrogen and/or carbon monoxide or a solid reductantcontaining a carbonaceous material.

[0019] In the method for making surface-coated reduced iron, preferably,the coating step is performed by controlling the temperature of thereduced iron at 250° C. or less.

[0020] In another aspect of the present invention, in a method formelting the surface-coated reduced iron including surface-coated reducediron in the form of powder and/or chips in a smelting furnace to producemolten iron, the surface of the reduced iron being coated with a taremulsion, the method includes the step of injecting the surface-coatedreduced iron in the form of powder and/or chips into a molten iron bathformed in the smelting furnace and/or into a slag formed on the molteniron bath.

[0021] In the method for melting the surface-coated reduced iron,preferably, the injecting step is performed using a gaseous medium.

[0022] As described above, the present invention is characterized bythat in order to prevent the oxidation of reduced iron, the surfacethereof is coated with a tar emulsion (emulsified tar).

[0023] The tar emulsion is produced by adding a small amount of thesurfactant and an appropriate amount of water to the hydrocarbon-basedmaterial, such as natural petroleum tar, coal tar, pitch, asphalt,liquefied coal, and residual oil from petroleum refining, followed bysuspending (emulsifying) the hydrocarbon-based material. The viscosityof the tar emulsion is significantly lower than the viscosity of thehydrocarbon-based material itself. Therefore, the tar emulsion formshomogeneous and flawless protective films even if the film thickness issmaller compared to the graphite and other inorganic films describedabove. Moreover, the tar emulsion enters the numerous open pores ofspongy reduced iron or the like and forms films on the inner surfaces ofthe open pores, thereby effectively preventing the surface oxidation ofthe reduced iron. When the organic film-forming material described aboveis used, the material must be dissolved or dispersed in a solvent todecrease the viscosity. However, since the tar emulsion contains thesurfactant and water, handling is significantly facilitated, and thecosts are also greatly reduced. As the tar emulsion, any one of thehydrocarbon-based materials described above may be used, and asnecessary, two or more of these materials may be used. In order todecrease the viscosity of the tar emulsion and in order to perform theemulsifying step while the content of the hydrocarbon-based material iskept high to a certain extent, preferably, the tar emulsion contains 60to 80 percent by mass of the hydrocarbon-based material, 0.1 to 1percent by mass of the surfactant, and the balance being substantiallywater.

[0024] The reduced iron to be coated with the tar emulsion is notparticularly limited to its production process. For example, the reducediron may be produced by being reduced by a gaseous reductant containinghydrogen and/or carbon monoxide in a shaft reduction furnace, a fixedbed reduction furnace, a fluidized bed reduction furnace, or the like.Alternatively, the reduced iron may be produced by being reduced by asolid reductant containing a carbonaceous material in a rotary hearthreduction furnace, a rotary kiln reduction furnace, or the like. Theform and/or shape of the reduced iron are not particularly limited. Forexample, the reduced iron may be in the form of pellets, lumps, sheets,and the like, as it is reduced, or may be in the form of briquettesformed by hot pressing (hot briquetting) after reduction, or may be inthe form of powder and chips generated by handling during or afterreduction. A mixture of two or more of these forms may be acceptable.

[0025] Since the tar emulsion has a sufficiently low viscosity atambient temperatures and can be thinly and homogeneously applied to thesurface of the reduced iron as it is, any conventional method may beapplied to coat the surface of the reduced iron with the tar emulsion.For example, the reduced iron may be immersed in the tar emulsion, orthe reduced iron may be sprayed with the tar emulsion. After theapplication of the tar emulsion, the water in the tar emulsion may beremoved by drying by heating (with heat), solar drying, or the like, asdesired.

[0026] When the surface of the reduced iron is coated with the taremulsion, the temperature of the reduced iron is controlled, preferably,at 250° C. or less, and more preferably, between 100 and 250° C. Bycontrolling the temperature of the reduced iron at 250° C. or less, thewater in the coated tar emulsion is not immediately removed byvaporization due to the sensible heat of the reduced iron, and theviscosity of the tar emulsion is kept low for a certain period of time,and thereby the surface of the reduced iron is easily coated with thetar emulsion thinly and homogeneously during that period. By controllingthe temperature of the reduced iron at 100° C. or more, after thereduced iron is coated with the tar emulsion, the water in the taremulsion is entirely or partially removed by vaporization, and therebydrying by heating (with heat), solar drying, or the like is notnecessary after coating, or the time of drying is shortened. As aresult, the cost of drying facilities can be minimized and the area ofthe yard for solar drying can be decreased.

[0027] With respect to the reduced iron of which surface is coated withthe hydrocarbon-based material after the water is removed as describedabove, homogeneous and flawless protective films are formed even if thethickness is small, the inner surfaces of many open pores are alsocoated, and thereby the surface oxidation of the reduced iron issignificantly and effectively prevented.

[0028] The amount of hydrocarbon-based material in the tar emulsion tobe deposited may be appropriately adjusted depending on the openporosity of the reduced iron, the temperature conditions duringtransportation and storage, etc. The amount of hydrocarbon-basedmaterial to be deposited is generally in the range of 0.01 to 10 percentby mass of the reduced iron, and more generally, in the range of 0.1 to5 percent by mass. That is, if the amount of coating is insufficient,the function as the oxidation-preventing film is not exhibitedsufficiently, and when the amount of coating is excessive, theoxidation-preventing effect is not increased, resulting in heat loss inthe step of melting the reduced iron or the large amount of pyrolyticgas generation.

[0029] When the surface-coated reduced iron is melted in a smeltingfurnace, such as a converter or an electric furnace, to produce molteniron (e.g., hot pig iron, and molten steel), the film of thehydrocarbon-based materials is easily pyrolyzed and the carbon residuecontributes to the reduction of the residual iron oxides in the reducediron and to the carburization of the molten iron. On the other hand, thepyrolytic gas generated is removed to exhaust gas, thus not adverselyaffecting the composition of the molten iron. Since the film iselectrically conductive, melting with an electric furnace or the like isnot adversely affected.

[0030] When the surface-coated reduced iron contains the iron in theform of powder and chips, the portion of powder and chips is meltedpreferably by being injected into the molten iron bath or into the slagformed on the molten iron bath. Thereby, it is possible to prevent thefly loss of dust into the exhaust gas, which occurs when the entire feedmaterial is fed by gravity from the top of the smelting furnace as iscommonly performed, thus improving the yield of iron. In addition, COgas, which is generated by the reduction of the iron oxides by thecarbon residue, stirs the iron bath and creates foamy slag so as toaccelerate the melting greatly, resulting in a significant reduction inthe melting time. The portion of powder and chips is preferably injectedby a gaseous medium via lances and tuyeres provided on the smeltingfurnace. Thereby, since the gaseous medium is added to the CO gasgenerated by the reduction, stirring of the molten iron and foaming ofthe slag are further activated. As the gaseous medium, an inert gas,such as N₂ or Ar, or a reducing gas, such as CO, H₂, CH₄, or a mixtureof these gases, is preferably used. Thereby, the molten iron isprevented from being decarburized and the melting is accelerated. Inaddition, since heat (electric power) required for melting is greatlyreduced, the production cost for molten iron (e.g., hot pig iron, andmolten steel) can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a process chart showing the steps of producingsurface-coated reduced iron in an embodiment of the present invention;and

[0032]FIG. 2 is a graph showing a change in the metallization over timewith respect to the individual examples of reduced iron in a weatherresistance test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033]FIG. 1 is a process chart showing the steps of producingsurface-coated reduced iron in an embodiment of the present invention.As shown in FIG. 1, in step 1, reduced iron A in the form of pellets orthe like is produced by reduction in a shaft reduction furnace 1 a or arotary hearth controlling furnace 1 b. The reduced iron A may besubjected to hot pressing (hot briquetting) to form briquettes in step2. In step 3, the reduced iron A, in the form of pellets or the like asproduced or in the form of briquettes, is cooled to 250° C. or less byinjection of inert gas, indirect water cooling, direct water spraying,or the like. In step 1, reduced iron in the form of powder may beproduced using a fluidized bed controlling furnace (not shown in thedrawing) instead of the shaft controlling furnace 1 a or the rotaryhearth controlling furnace 1 b, and the reduced iron in the form ofpowder may be formed into briquettes by hot pressing (hot briquetting)in step 2.

[0034] The reduced iron A in the form of pellets or the like, or in theform of briquettes is transferred to step 4, in which the surface of thereduced iron A is coated with a tar emulsion B. In step 4, for example,as shown in FIG. 1, a narrow container 4 a provided with a screw feeder4 b is filled with the tar emulsion B in an appropriate quantity, thereduced iron A is continuously fed from the inlet into the container 4 aand is immersed in the tar emulsion B for a predetermined period by thescrew feeder 4 b, and the reduced iron A is then discharged from theoutlet of the container 4 a. As the tar emulsion B, for example, anemulsified natural petroleum tar, which is commercially available asfuel for thermal powder generation, containing approximately 70 percentby mass of a natural petroleum tar, 0.3 to 0.5 percent by mass of asurfactant, and the balance being substantially water, may be used. Asnecessary, an appropriate amount of water may be added thereto to adjustthe concentration of the hydrocarbon-based material.

[0035] By appropriately changing the concentration of the tar emulsionB, the temperature of the reduced iron A to be immersed into the taremulsion B, the immersing time, etc., it is possible to adjust theamount of the tar emulsion B to be deposited on the reduced iron A.

[0036] Additionally, if reduced iron A cooled to 250° C. or less isused, instead of the immersion method described above, a method may beused in which the tar emulsion A is sprayed on the reduced iron A whilethe reduced iron A is moved by a belt conveyor. In such a method, theamount of the tar emulsion B to be deposited on the reduced iron A canbe adjusted by the concentration of the tar emulsion B, the temperatureof the reduced iron, the spraying time, etc.

[0037] Additionally, step 3 (reduced iron-cooling step) and step 4 (taremulsion-applying step) are not necessarily performed continuously. Thereduced iron A may be completely cooled to ambient temperature in step3, and after storing for a certain period or after transportation,coating treatment may be performed in step 4.

[0038] Next, reduced iron coated with the tar emulsion (surface-coatedreduced iron) A₁ is transferred to step 5 to dry/remove the water in thetar emulsion. In step 5, for example, the surface-coated reduced iron A₁is placed in a layer on a moving, endless grate, and a drying gas ofwhich temperature is set at 90 to 200° C. is passed through the layer ofthe surface-coated reduced iron A₁. As the drying gas, for example, anexhaust gas produced by burning fuel, such as natural gas, heavy oil, orpulverized coal, may be used. Alternatively, exhaust gas from thereducing furnace in step 1 may be used by mixing with air and adjustingthe temperature of the mixture. Additionally, by controlling its coolingtemperature in step 3 at a highest possible temperature below 250° C.,since the water in the tar emulsion B can be dried/removed by thesensible heat of the reduced iron A when the surface of the reduced ironA is coated with the tar emulsion B in step 4, step 5 may be omitted orreduced.

[0039] As described above, by removing the water by drying from thecoated tar emulsion, the hydrocarbon-based materials, such as a naturalpetroleum tar, remain on the surface of the reduced iron to form ahomogeneous protective film, and thereby even if the surface-coatedreduced iron is stored and transported in the air for a long period oftime, reoxidation can be prevented.

[0040] The reduced iron A is subjected to various types of mechanicalhandling as the reduced iron A is transported from step 1 to step 5 inthat order, and therefore the parts of the reduced iron A are broken toform power and chips. In the conventional reduced iron without surfacecoating, the powder and chips are easily reoxidized due to the largespecific surface areas. Consequently, the metallization is decreased,and the quality is deteriorated. Heat generation by the oxidation mayalso result in ignition and firing. Therefore, the powder and chips mustbe removed by sieving or the like before storage and transportation,resulting in a decrease in the yield of reduced iron in the conventionalmethod. In addition, the unit and apparatus for reusing the removedpowder and chips must also be prepared separately. In contrast, in thepresent invention, the entire amount of reduced iron can be transferredto the tar emulsion-coating step without sieving. That is, although thesurface-coated reduced iron produced through the tar emulsion-coatingstep contains powder and chips in addition to lumpy shaped reduced iron,such as pellets and briquettes, since the surfaces of the powder and thechips are also densely coated, their reoxidation is prevented, and theproblems associated with the conventional reduced iron are not arisen.

[0041] When the surface-coated reduced iron is melted in a smeltingfurnace, such as a converter or an electric furnace, to produce hot pigiron and molten steel, although the entire coated reduced iron togetherwith powder and chips may be charged or poured by gravity from the topof the smelting furnace, a large amount of fly dust loss may begenerated, the same as the conventional method. Therefore, preferably,by sieving out the powder and chips, only lumpy reduced iron is fed bygravity from the top of the smelting furnace, and the powder and chipsare separately injected into the iron bath and the slag layer retainedin the smelting furnace. Specifically, for example, a predeterminedamount of powder and chips is discharged from a storage bin for storingthe powder and chips, and is transferred to an injection hopper. Thepowder and chips are then directly injected into the iron bath or theslag layer via injection lances immersed in the iron bath or the slaglayer or through tuyeres provided on the side wall or bottom of thesmelting furnace, using high-pressure N₂ or high-pressure reduction gaswhich is produced by cooling a part of reduction gas for the reducingfurnace, followed by pressurization. Alternatively, the powder and chipsmay be injected into the slag layer via injection lances provided withinjection ports above the slag layer. By the direct injection into theiron bath or the slag layer or by the high-speed injection, even thepowder is effectively caught by the iron bath and the slag layer, andthereby the fly loss of dust is significantly decreased. As describedabove, the surface-coated reduced iron in the form of powder and chipsthus injected into the iron bath or the slag layer stirs the iron bathand foams the slag, and the melting rate of the reduced iron is greatlyaccelerated, resulting in a significant reduction in the melting time,thus significantly increasing the productivity.

[0042] The present invention will be described in more detail based onthe examples.

EXAMPLE 1

[0043] In order to confirm the reoxidation-preventing effect inaccordance with the present invention, with respect to reduced iron ofwhich surface was coated with an emulsified natural petroleum tar(Example 1 of the present invention), reduced iron of which surface wascoated with an organic film-forming material (Comparative Example 1),and untreated reduced iron without surface coating (Comparative Example2), a weather resistance test was conducted under the same conditions.

[0044] As the emulsified natural petroleum tar, Orimulsion (registeredtrademark) manufactured by Bitumenes Orinoco, S.A. was used. Theemulsified natural petroleum tar contained, in percent by mass, 71±1% ofa natural petroleum tar, 0.3 to 0.5% of a surfactant, and the balancebeing water, and had the following composition determined by elementaryanalysis: 59.0 to 60.5% of C, 7.2 to 7.8% of H, 0.43 to 0.56% of N, and2.1 to 2.9% of S. The viscosity of the emulsified natural petroleum tarwas 0.9 Pa·s or less at 30° C. In a container filled with the emulsifiednatural petroleum tar, reduced iron pellets (with an average particlesize of 12 mm; powder and chips with a particle size of 6 mm or lessbeing removed beforehand) produced with a shaft reduction furnace andcooled to ambient temperature was immersed. The surface-coated reducediron pellets were collected from the container and kept for 10 minutesin a drier of which temperature was set at 90° C. to dry and remove thewater. The amount of the hydrocarbon-based materials deposited afterdrying was approximately 3 percent by mass of the reduced iron.

[0045] On the other hand, with respect to surface coating using theorganic film-forming material, reduced iron pellets prepared in the samemanner as described above were immersed in a solution prepared by adding50 parts by mass of pitch to 50 parts by mass of kerosene and dissolvingthe pitch homogeneously. The surface-coated reduced iron pellets werecollected from the solution and drying was performed under reducedpressure to remove the kerosene by vaporization. The amount of theorganic film-forming material deposited was approximately 3 percent bymass of the reduced iron.

[0046] In order to perform the weather resistance test under the sameconditions as the actual long-term storage conditions, the individualtest materials (reduced iron pellets) were stored in outdoor and wereexposed to sunshine and rain at a reduced iron manufacturing plant inVenezuela. The test materials were placed in one layer in a bat providedwith drain holes on the bottom. As for the climatic condition, theaverage annual precipitation was 911 mm, the average humidity was 76%,and the temperature was 17 to 41° C. A sample in a small quantity wastaken from each test material every 5 days, and the total iron (T. Fe)and the metallic iron (M. Fe) were determined by chemical analysis. Themetallization (M. Fe/T. Fe×100%) was calculated to find a change in themetallization over time for each test material. The results thereof areshown in FIG. 2. As is obvious from FIG. 2, with respect to the reducediron pellets of which surfaces were coated with the organic film-formingmaterial in Comparative Example 1, the decrease in the metallization wassmaller and an reoxidation-preventing effect was exhibited compared tothe untreated reduced iron pellets in Comparative Example 2. However,with respect to the reduced iron of which surface was coated with theemulsified natural petroleum tar in Example 1 of the present invention,the decrease in the metallization was much smaller and a largerreoxidation-preventing effect was exhibited. As is also clear from FIG.2, compared to the initial metallization of 92% in the individual testmaterials at the start of the weather resistance test, after 30 days,the metallization of Comparative Example 2 decreased to 62%, and themetallization of Comparative Example 1 decreased to 78%, while Example 1of the present invention retained the metallization of 87% which wassufficient for use in a smelting furnace in the subsequent step.

EXAMPLE 2

[0047] In order to confirm the effect of the method for melting thesurface-coated reduced iron in accordance with the present invention, amelting test for surface-coated reduced iron was performed using anelectric arc furnace as the smelting furnace.

[0048] In a container filled with the same emulsified natural petroleumtar as that used in Example 1 of the present invention, reduced ironcontaining powder and chips, which was produced in a shaft reductionfurnace and cooled to ambient temperature was immersed. Thesurface-coated reduced iron was collected from the container and keptfor 10 minutes in a drier of which temperature was set at 90° C. to dryand remove the water, and thus Sample 1 was prepared. A half of Sample 1was separated by sieving into reduced iron in the form of powder andchips having a particle size of less than 6 mm and reduced iron in theform of lumps (mainly pellets) having a particle size of 6 mm or more,and thus sample 2 was prepared.

[0049] An iron bath and a slag layer were preliminarily formed in theelectric arc furnace. With respect to the case in which the reduced ironof Sample 1 which was not sieved was continuously fed by gravity intothe electric arc furnace from above the iron bath (Comparative Example3) and the case in which reduced iron lumps having the particle size of6 mm or more were continuously fed by gravity from above the iron bathand the powder and chips having the particle size of less than 6 mm werecontinuously injected by N₂ gas through injection lances immersed in theiron bath (Example 2 of the present invention), the melting time and thepower consumption required for melting the entire amount of the sampleswere measured.

[0050] As a result, it has been confirmed that the melting time and thepower consumption for Example 2 of the present invention weresignificantly decreased compared to Comparative Example 3. Additionally,by visual observation, it has been confirmed that the fly loss of dustin the melting method in Example 2 of the present invention wassignificantly decreased compared to the melting method in ComparativeExample 3.

What is claimed is:
 1. A surface-coated reduced iron having the surfacecoated with a tar emulsion.
 2. The surface-coated reduced iron accordingto claim 1, wherein said tar emulsion comprises at least onehydrocarbon-based material selected from the group consisting of naturalpetroleum tar, coal tar, pitch, asphalt, liquefied coal, and residualoil from petroleum refining; a surfactant; and water.
 3. Thesurface-coated reduced iron according to claim 2, wherein said taremulsion comprises 60 to 80 percent by mass of the hydrocarbon-basedmaterial, 0.1 to 1 percent by mass of the surfactant, and the balancebeing substantially water.
 4. A method for making a surface-coatedreduced iron comprising the step of coating the surface of a reducediron with a tar emulsion.
 5. The method for making the surface-coatedreduced iron according to claim 4, wherein said tar emulsion comprisesat least one hydrocarbon-based material selected from the groupconsisting of natural petroleum tar, coal tar, pitch, asphalt, liquefiedcoal, and residual oil from petroleum refining; a surfactant; and water.6. The method for making the surface-coated reduced iron according toclaim 5, wherein said tar emulsion comprises 60 to 80 percent by mass ofthe hydrocarbon-based material, 0.1 to 1 percent by mass of thesurfactant, and the balance being substantially water.
 7. The method formaking the surface-coated reduced iron according to claim 4, whereinsaid reduced iron is produced by reduction with a gaseous reductantcontaining hydrogen and/or carbon monoxide or a solid reductantcontaining a carbonaceous material.
 8. The method for making thesurface-coated reduced iron according to claim 4, wherein said coatingstep is performed by controlling the temperature of said reduced iron at250° C. or less.
 9. A method for melting a surface-coated reduced ironhaving the surface coated with a tar emulsion in a smelting furnace, thesurface-coated reduced iron comprising surface-coated reduced ironpowder and/or chips, the method comprising the step of injecting thesurface-coated reduced iron powder and/or chips into a molten iron bathformed in the smelting furnace and/or into a slag formed on the molteniron bath.
 10. The method for melting the surface-coated reduced ironaccording to claim 9, wherein said injecting step is performed using agaseous medium.